Various fasteners that releasably interengage with other articles are widely used as low cost garment fasteners, such as on disposable diapers. One type of headed stem fastener is the hook portion of a hook-and-loop fastener. Although the hook portion of a hook-and-loop fastener is typically designed to engage with a loop strip, the hook portion can be used by itself to become releasably fastened to fabrics that are easily penetrated by the hook. Another type of headed stem fastener that is particularly suited for this purpose is a mushroom-headed fastener, such as available under the product designation XMH-4152 from Minnesota Mining & Manufacturing Company of St. Paul, Minn. Mushroom-headed fasteners can be designed to become releasably fastened to burlap, Terri cloth, and tricot.
Stem fasteners are typically formed by capping polymeric stems extending distally from a backing layer. The precursor web containing the stems can be prepared according to a variety of techniques, such as disclosed in U.S. Pat. No. 4,290,174 (Kalleberg) and U.S. Pat. No. 5,679,302 (Miller), entitled Mushroom-Type Hook Strip for a Mechanical Fastener (WO 94/23610).
FIGS. 1 and 3 are schematic illustrations of two commonly used methods for capping stems projecting upward from a precursor web. In the embodiment of FIG. 1, a precursor web 20 is fed through a gap in a nip 21 between two calender rolls 22 and 24. The heated calender roll 22 contacts a predetermined portion of a distal end 26 of the stems 28 projecting upward from a backing 30. The temperature of the heated calender roll 22 is maintained at a temperature that will readily deform the distal ends 26 under mechanical pressure in the nip 21.
Maintaining the distal ends 26 at this temperature allows melting and molecular disorientation of the stems 28. During such contact and/or upon subsequent cooling, a head 32 is formed on the distal ends 26. The heads 32 can be a variety of shapes, such as mushroom-shaped heads, "umbrella," "nail head," "golf tee" and "J-shaped." Mushroom shaped heads typically have a flat, planar or slightly convex upper surface and a maximum cross-section larger than the diameter of the stem immediately below the head (see FIGS. 8A and 8B).
The capping mechanism is generally a time-temperature-pressure phenomenon, although it is possible that some heat may be transmitted to the stems by convection. In practice, the height of the stems 28 and the finished height of the capped stem 32 are determined by the product design. The upper temperature at the roll 22 is generally limited to the temperature at which the polymer of the stems 28 sticks to the roll.
FIG. 2 is a diagram illustrating the size of the capping surface 34 (see FIG. 1) of a conventional calendering system. In FIG. 2, R is the radius of the heated roll, X is the distance over which the precursor web 20 is capped, t.sub.2 is the height of the capped stem 32, and t.sub.1 is the height of the stem 28. For a typical product, t.sub.2 is approximately 0.51 mm and t.sub.1 is approximately 0.74 mm. Using the following equation, the capping surface or distance 34 for a calender roll with a diameter of 45.7 cm (18 inches) is approximately 7.2 mm. ##EQU1##
FIG. 3 is a schematic illustration of an alternate method and apparatus for forming headed stems 42. The precursor web 20 is positioned so that a heated platen 40 is located above the stems 28. The heated platen 40 heats the air near the distal ends 26 of the stems 28 to cause the ends to soften by convection. The stems are deformed into generally hemispherical-shaped heads 42. In order to achieve controlled deformation of the distal ends 26, the temperature at which the heated platen 40 can be operated is limited by the polymer from which the stems 28 are constructed. Additionally, the ability to control the shapes of the heads 42 is limited.